Project Summary Telomeres are evolutionarily conserved protein-DNA complexes at the physical ends of linear eukaryotic chromosomes. Telomeres shorten with age in most human cells, and their initial length pre-determines cellular lifespan. Mutations in telomere maintenance genes lead to cancer, premature aging and a number of age-related disorders. While mean telomere length in humans shows considerable inter-individual variation and appears to be under strong genetic control, the exact nature of factors establishing telomere length set point remains elusive. In our preliminary results using the model plant Arabidopsis thaliana we identify a major effect QTL in a recombinant inbred population that explains 48% of telomere length variation and map this QTL to a candidate gene, NOP2A. Notably, expression of the human NOP2 ortholog is linked to tumorigenesis and serves as a prognostic marker of tumor development. In this proposal, we will utilize genetic, genomic, biochemical and epigenetic approaches to decipher the mechanism of AtNOP2A function, and to uncover additional genetic and epigenetic factors involved in telomere length control. In Aim 1, through a series of quantitative transgenic rescue experiments we will identify the causal SNP and explore the mechanism by which NOP2A impacts telomere length. We will also employ powerful Arabidopsis genetic, genomic and transcriptomic tools to identify and characterize NOP2A-dependent genes and trans-regulators. In Aim 2, we will perform GWAS in 1,001 Arabidopsis genotypes and fine-map additional QTL in a bi-parental Arabidopsis RIL population to identify novel polymorphisms that affect telomere length. We will then perform a series of knock-out and transgenic rescue experiments to functionally characterize candidate genes and validate their role in telomere biology. In Aim 3, we will utilize a unique A. thaliana epigenetic recombinant inbred population with almost identical DNA sequences, but variable methylation and gene expression profiles, to fine-map two previously identified large-effect epi-QTL governing telomere length, and analyze how heritable epigenetic variation directly affects telomere length. Overall, the results of this study are expected to significantly increase understanding of genetic differences underlying telomere length polymorphism in natural Arabidopsis populations. Because modes of telomere regulation are highly conserved, our data may also provide novel insight into the molecular basis for different rates of aging and predisposition to diseases associated with telomere abnormalities in humans.